Equilibrium shifting

According to Le Chatelier s principle, a system at equilibrium adjusts so as to mini mize any stress applied to it When the concentration of water is increased the system responds by consuming water This means that proportionally more alkene is converted to alcohol the position of equilibrium shifts to the right Thus when we wish to pre pare an alcohol from an alkene we employ a reaction medium m which the molar con centration of water is high—dilute sulfuric acid for example... 

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH2). With little or no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH3+) the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large increase in the number of protonated amine species recorded (spectrum b). 

Stratifying water systems for selective extraction of thiocyanate complexes of platinum metals have been proposed. The extraction degree of mthenium(III) by ethyl and isopropyl alcohols, acetone, polyethylene glycol in optimum conditions amounts to 95-100%. By the help of electronic methods, IR-spectroscopy, equilibrium shift the extractive mechanism has been proposed and stmctures of extractable compounds, which contain single anddouble-chai-ged acidocomplexes [Rh(SCN)J-, [Ru(SCN)J, [Ru(SCN)J -have been determined. Constants of extraction for associates investigated have been calculated. 

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

FIGURE 15.9 Monod-Wyman-Changeux (MWC) model for allosteric transitions. Consider a dimeric protein that can exist in either of two conformational states, R or T. Each subunit in the dimer has a binding site for substrate S and an allosteric effector site, F. The promoters are symmetrically related to one another in the protein, and symmetry is conserved regardless of the conformational state of the protein. The different states of the protein, with or without bound ligand, are linked to one another through the various equilibria. Thus, the relative population of protein molecules in the R or T state is a function of these equilibria and the concentration of the various ligands, substrate (S), and effectors (which bind at f- or Fj ). As [S] is increased, the T/R equilibrium shifts in favor of an increased proportion of R-conformers in the total population (that is, more protein molecules in the R conformational state). 

This reaction is driven to the right in tissues by the high COg concentration the equilibrium shifts the other way in the lungs where [COg] is low. Thus, car-bamylation of the N-termini converts them to anionic functions, which then form salt links with the cationic side chains of Arg al41 that stabilize the deoxy or T state of hemoglobin. 

Those steps are reversible reactions, with the equilibrium shifted to the left. However the overall reaction can be carried out in good yield since the /3-ketoester 2 is converted into the conjugate base 6 by the lost alkoxide 5 the ester is more acidic than the alcohol R OH 7 ... 

Conversion of Amides into Carboxylic Acids Hydrolysis Amides undergo hydrolysis to yield carboxylic acids plus ammonia or an amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of add chlorides or esters but the mechanisms are similar. Acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by transfer of a proton from oxygen to nitrogen to make the nitrogen a better leaving group and subsequent elimination. The steps are reversible, with the equilibrium shifted toward product by protonation of NH3 in the final step. 

Basic hydrolysis occurs by nucleophilic addition of OH- to the amide carbonyl group, followed by elimination of amide ion (-NH2) and subsequent deprotonation of the initially formed carboxylic acid by amide ion. The steps are reversible, with the equilibrium shifted toward product by the final deprotonation of the carboxylic acid. Basic hydrolysis is substantially more difficult than the analogous acid-catalyzed reaction because amide ion is a very poor leaving group, making the elimination step difficult. 

The application of this principle to several different systems is shown in Table 12.7. In system 2, the number of moles of gas decreases from to 1 as the reaction goes to the right. Hence increasing the pressure causes the forward reaction to occur a decrease in pressure has the reverse effect. Notice that it is the change in the number of moles of gas that determines which way the equilibrium shifts (system 4). When there is no change in die number of moles of gas (system 5), a change in pressure has no effect on the position of the equilibrium. 

A bacterial isolate APN has been shown to convert a-aminopropionitril enantioselectively to L-alanine (94% yield, 75% e e). However, the major disadvantage of this approach, is the low stability of most aminonitriles in water (for example a-aminophenylacetonitrile in water of pH 7, degrades completely within 48 hours). The aminonitriles are always in equilibrium with the aldehyde or ketone and ammonia/HCN. Polymerisation of hydrogen cyanide gives an equilibrium shift resulting in the loss of the aminonitrile. Therefore, a low yield in amino adds is to be expected, which makes this method less attractive for the industrial synthesis of optically active amino adds. 

Because P°/RT is a constant, for this expression to remain constant when the volume (V) of the system is reduced, the ratio n,o4Awno,)2 must increase. That is, the amount of NO, must decrease and the amount of N204 must increase. Therefore, as we have seen before, as the volume of the system is decreased, the equilibrium shifts to a smaller total number of gas molecules. 

FIGURE 13.27 (a) The activation energy for an endothermic reaction is larger in the forward direction than in the reverse and so the rate of the forward reaction is more sensitive to temperature and the equilibrium shifts toward products as the temperature is raised, (b) The opposite is true for an exothermic reaction, in which case the reverse reaction is more sensitive to temperature and the equilibrium shifts toward reactants as the temperature is raised. 

Suppose this reaction is occurring in a CSTR of fixed volume and throughput. It is desired to find the reaction temperature that maximizes the yield of product B. Suppose Ef > Ef, as is normally the case when the forward reaction is endothermic. Then the forward reaction is favored by increasing temperature. The equilibrium shifts in the desirable direction, and the reaction rate increases. The best temperature is the highest possible temperature and there is no interior optimum. 

As pH increases, the concentration of OH also goes up. H+ ions are removed to form water, the equilibrium shifts from left to right, and more relatively nonpolar RNH2 is generated. 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

The volumetric ratio of the two liquid phases (j6 = Forg/ Faq) can affect the efficiency of substrate conversion in biphasic media. The biocatalyst stability and the reaction equilibrium shift are dependent on the volume ratio of the two phases [29]. In our previous work [37], we studied the importance of the nonpolar phase in a biphasic system (octane-buffer pH 9) by varying the volume of solvent. The ratio /I = 2/10 has been the most appropriate for an improvement of the yield of the two-enzyme (lipase-lipoxygenase) system. We found that a larger volume of organic phase decreases the total yield of conversion. Nevertheless, Antonini et al. [61] affirmed that changes in the ratios of phases in water-organic two-phase system have little effect upon biotransformation rate. 

Possibility of significant equilibrium shift in the desired direction. Thermodynamically unfavorable reactions become possible. 

Observing and Inferring Describe the stress that caused an equilibrium shift when the bottle of club soda was opened. 

It is noteworthy that all these reactions are reversible, but under reducing conditions the equilibrium shifts to the right-hand side, and under oxidizing conditions it shifts to the left-hand side. For all resulting binuclear ions at rather low concentration in solutions, reactions take place, accompanied by the rupture of the M-M bond and the formation of unstable mononuclear complexes, which are readily oxidized even by the solvent. At high concentrations of binuclear ions cycloaddition reactions of multiple M-M bonds (these reactions are considered in section 3.5) occur to form stable, under the given conditions, polynuclear clusters. 

However, if there was sufficient amount of H+ ions in the solution, equilibrium shifted towards the left and bismuth remained in the solution as Bi3+. Therefore, during sonication activated water molecules were formed which reacted with Bi3+ ions. But instead of forming Bi(OH)3, they formed bismuthyl ions, BiO+, due to the fact that Bi(OH)3 was a weaker base, therefore hydrolysed readily to generate bismuthyl ion, BiO+. These steps could be summarised as under ... 

Before addition of the extra hydrogen, its concentration was 0.250 mol/L. After addition of the extra hydrogen, but before any equilibrium shift could take place, there would be 0.253 mol/L. The shift of the equilibrium would use up some but not all of the added hydrogen, and so the final hydrogen concentration must be above 0.250 mol/L and below 0.253 mol/L. Some nitrogen has been used up, and its final concentration must be less than its original concentration, 0.100 mol/L. Some additional ammonia has been formed, and so its final concentration has been increased over 0.200 mol/L. Notice that Le Chatelier s principle does not tell us how much of a shift there will be, but only qualitatively in which direction a shift will occur. 

Write an equation for the addition of heat to a water-ice mixture at 0°C to produce more liquid water at 0°C. Which way does the equilibrium shift when you try to raise the temperature ... 

The equilibrium shifts toward liquid water as you add heat in an attempt to raise the temperature. (However, the temperature does not change until all the ice is melted and this equilibrium system is destroyed.)... 

Hydrogen gas is added to an equilibrium system of hydrogen, nitrogen, and ammonia. The equilibrium shifts to reduce the stress of the added hydrogen. How much hydrogen will be present at the new equilibrium compared with the old equilibrium—more, less, or the same concentration ... 

There will be more hydrogen and more ammonia present at the new equilibrium, and less nitrogen. The concentration of hydrogen is not as great as the original concentration plus that which would have resulted from the addition of more hydrogen, however. Some of the total has been used up in the equilibrium shift. 

The solution of HC2H302 and C2H302 in H20 results in the relative quantities of each of the species in the equation as shown under the equation. If H30+ is added to the equilibrium system, the equilibrium shifts to use up some of the added H30+. If the acetate ion were not present to take up the added H30+, the pH would drop. Since the acetate ion reacts with much of the added H, 0+, there is little increase in H30+ and little drop in pH. If OH- is added to the solution, it reacts with the H,0+ present. But the removal of that H30+ is a stress, which causes this equilibrium to shift to the right, replacing much of the H30+ removed by the OH-. The pH does not rise nearly as much in the buffered solution as it would have in an unbuffered solution. 

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